Selective zonal isolation of oil wells

ABSTRACT

In the horizontal parts of deviated wells arranged within a producing formation, good communication between the formation and the wellbore liner is required, and this may be achieved by using a slotted or perforated liner in the horizontal section without any cementing to bond the liner to the wellbore. However, problems can arise if it is desired to work selectively in a zone in the horizontal section, since with a perforate liner there is no way of isolating the zone from the remainder of the well by using internal packers. This problem can be overcome by providing packers around the outside of the liner, this then allows packers inside the liner to be used to isolate a portion of the well. However, these external casing packers must be sealed against the wellbore, they must be positioned in advance, and they substantially increase the cost of the completion. The present invention suggests novel means of zonal isolation in a well with a perforate liner, this isolation being achieved by using a liquid composition that can be pumped into the relevant annular region between the liner and the wellbore and then set to form a plug that prevents communication from one side to the other except via the liner. More specifically, the invention provides a method of placing a plug around a perforate liner in which: a pair of spaced packers (14, 16) are placed inside the liner (12) to define both a chamber (20) inside the liner which includes a perforation (22) and also a region (24) to be plugged outside the liner, a predetermined volume of plugging fluid is pumped into the chamber and, via the perforation, into the region to be plugged; and after the plugging fluid has set the packers are removed from the liner.

The present invention relates to a method of selectively isolating zonesof oil wells or the like in which a liner or casing is situated in thewell but is not continuously cemented to the wellbore wall so as to beisolated therefrom.

In conventional well completion a tubular liner or casing is run intothe well after it is drilled, and cement is pumped between the casingand the wellbore wall and allowed to set. This isolates the variouszones of the well from each other, and so prevents, for example, oilfrom entering aquifers which might be at lower pressure and providingdrinking water. Where it is desired to allow formation fluids such asoil or gas to enter the well, the lining and cement are perforated by anexplosive charge in order to provide a channel for the fluid to enterthe lining so that it can pass more sensitive zones without causingdamage or pollution.

This technique generally works well in wells which are more or lessvertical, since it is straightforward to run the liner into the well, toensure it is centralised, and to ensure that cement is placed evenlyaround the liner and bonds to both the liner and the wellbore. However,when the well is deviated from vertical--particularly when highlydeviated, even horizontal--certain problems can occur. In particular, itis often difficult to ensure that the liner remains central in the wellbefore the cement is placed. There is a tendency for the liner to lie onthe lower side of the well such that when the cement is pumped it flowseasily along the high side of the liner, but little if any penetratesaround the low side; this can lead to bonding problems. Also, the toolsused to perforate the liner and cement can become eccentred, and then donot operate efficiently.

The horizontal parts of deviated wells are often arranged so as toremain within a producing formation, and therefore in these sectionsgood communication between the formation and the liner is requiredwhereas in the vertical sections leading to the surface good zonalisolation is required. One technique which is used in such horizontalsituations is to provide a perforate liner--for instance, a slotted,perforated, or predrilled liner, or a screen or a pre-packed screen--inthe horizontal section of the well without any cementing to bond theliner to the wellbore, the annular gap between the liner and thewellbore either being left "empty" or (as may be preferable in certaincircumstances) being packed with suitably-sized gravel. However,problems can arise if it is desired to perform a selective welltreatment on, or produce selectively from, one zone in the horizontalsection since inside a perforate liner there is no way of isolating thezone in question from the remainder of the well by using packers as isdone with a conventional liner, because fluid can by-pass the plug byexiting the liner into the annular space therearound. One techniquewhich has been proposed to overcome this problem is to provide one ormore packers around the outside of the liner (external casing packers)which contact the wellbore and provide localised restriction tocommunication outside the liner; this then allows packers inside theliner to be used to isolate a portion of the well. Unfortunately, thereare also problems associated with the use of external casing packers;they must be sealed against the wellbore, they must be positioned inadvance, and they substantially increase the cost of the completion.

U.S. Pat. No. 5,197,543 discloses a method of isolating regions of ahorizontal wellbore in an unconsolidated formation involving placementof alternating blank (unperforated) liner sections such as 32a, 32b andscreen sections, eg slotted liners, such as 30a 30b, 30c. Internal plugs42, 44 are positioned in blank liner sections adjacent the region to beplugged, and plugging liquid is pumped through the intervening screensection into the surrounding formation. The technique is only describedin connection with treatment of unconsolidated formation, where thesurrounding formation has collapsed around at least the blank linersections.

The present invention seeks to provide means of zonal isolation in awell with a perforate liner which can be positioned according torequirements; the invention proposes that this be achieved by using aliquid composition that can be pumped, into the relevant annular regionbetween the liner and the wellbore and there set to form a plug thatprevents communication from one side to the other except via the liner.

In one aspect, therefore, the present invention provides a method ofplacing a plug in a region around a perforate liner in a wellbore, themethod comprising:

placing a pair of packers inside the liner where the plug is to bepositioned, the packers being spaced apart so as to define a chamberinside the liner which includes a perforation and also a region to beplugged outside the liner; and

pumping a predetermined volume of plugging fluid into the chamber and,via the perforation, into the region to be plugged, which predeterminedvolume of plugging fluid is sufficient to displace substantially allother-fluids from the region to be plugged.

Although it may be desirable in some cases--for example, when abandoningthe section of the well beyond the emplaced plug--to leave the pair ofpipe-internal packers in position, most usually it will be appropriateto remove then to clean away the residue of plugging fluid remainingwithin the liner itself.

The volume of the chamber is preferably smaller than the volume of theregion to be plugged in order to reduce the amount of plugging fluidwhich must be cleaned from the liner after placement of the plug.

The perforate liner can take any suitable form. Typically, it is aslotted liner or a pre-packed screen.

The plugging fluid is typically a cement or the like which hasappropriate rheological properties to displace other fluids when pumpedinto the region and to remain there while it sets so as to seal againstthe wellbore and the casing and form an impermeable plug. Prior to itsuse, it may be desirable to pump a wash fluid through the chamber andregion to be plugged. These wash fluids and their pumping rates arewell-known in the field of cementing and well treatment, and aredesigned according to the particular nature of the job in hand.

The plugging fluid is conveniently pumped either from the surface to thechamber via a tube, or by means of a downhole pump from a reservoirlocated near the packers in the wellbore.

Most preferably the volume of fluid is such that when in the annulus itdoes not extend beyond the limit of the packers, but nevertheless it ispossible for the fluid to extend past them (although, to prevent thefluid then re-entering the liner, it is desirable that the criticalpressure drop along the annular region being plugged does not exceed thepressure drop across the slots in the liner--i.e. no fluid enters theliner beyond the packer).

Once the plugging fluid is in place in the annulus it will normally bethe case that the residue of the fluid in the chamber (and possibly inthe liner outside the packer pair) needs to be washed out. Again,suitable wash fluids and their pumping rates are well-known in the fieldof cementing and well treatment, and need no further comment here.

The plugging fluid is designed to meet various requirements--thus: toallow mixing and processing at the surface, and pumping through thetubing to the chamber; to ensure adequate placement into the region tobe plugged; to remain in position during set, and prevent re-entry ofthe fluid into the liner when the packers are moved; and to set toprovide an impermeable area in the region. The optimum fluid isthixotropic, where the characteristic gelling time of the fluid isshorter than or comparable to the time taken to displace the fluid intothe region, and the gel strength or viscosity of the material issufficient to eliminate gravity-induced flows. The required gel strengthand gelling time are calculated to achieve optimum displacement for thespecific geometry of the region, the pumping time and the densitydifference between the fluid and the oil/water initially filling theregion for each job. The plugging fluid should possess the followingproperties to be useful in this method:

1) It should be a setting system--i.e., fluid when pumped but capable ofchanging to a solid after it has been placed.

2) It should not slump under gravity--i.e., its gel strength isreasonably high. However, when the fluid is being pumped into the regiona degree of gravity-induced flow to the lower side is desirable sincethis is the region where placement is most difficult.

3) It must be sufficiently pumpable to be delivered through the tubingto the relevant site.

In order that the fluid can possess all of these requirements it ishighly desirable that it be thixotropic in nature, since during pumpingthe confining pressure keeps the fluid tight against the borehole wallsand liner surface but once pumping stops there are no containmentpressures so the plug must become substantially "self-supporting" veryquickly, so as not to move significantly along the annulus.

Examples of suitable fluids are: foamed cements; unfoamed cementscontaining smectic clays such as bentonite and attapulgite; unfoamedcements containing welan gum, aluminium and/or iron sulphate, and/orcalcium sulphate (gypsum) as thixotropy agents; thermosetting polymerssuch as epoxy, vinylester, phenolic and polyester resins; andcross-linking polymer gels (possibly with an added thixotrope).

An example of a particular suitable fluid, designed for a test in a onethird scale model of a typically 7" (about 17.5 cm) slotted liner, is:

    ______________________________________                                               Class A cement 720 pbw                                                        Gypsum          72 pbw                                                        Water          349 pbw                                                 ______________________________________                                    

Another example of a suitable fluid is:

    ______________________________________                                        Class G cement       792    pbw                                               Thixotrope*          0.05   pbw                                               Water                349    pbw                                               ______________________________________                                    

* The thixotrope was a mixture of 32.8% alnminium sulphate, 4.5% ferroussulphate, 3% sulphuric acid and 59.7% water, by weight.

It will be appreciated, of course, that it may be necessary to choose aplugging fluid of a type that is suited to the variety of liner beingemployed, for certain plugging fluids should not be used with certaintypes of liner--thus, if the liner is a pre-packed gravel screen then itwould be quite unsuitable to employ as the plugging fluid a conventionalcement composition, and instead one of the several resin fluids shouldbe used. Which plugging fluid is suitable for which liner variety willbe evident to those skilled in the art, but by way of guidance it can besaid that cement fluids should only be used where the liner perforationsare greater than about six times the maximum cement grain size.

After the plug is in place, and set, the only fluid flow which ispossible at the plug is through the liner (which can of course be sealedwith a packer if required).

After the plugging fluid has been pumped into the region to be plugged,the packers are desirably moved to a location separate from the regionto be plugged, and some suitable fluid, preferably a wash fluidspecifically designed to remove the plugging fluid, is circulatedthrough the chamber and liner to remove unwanted plugging fluidtherefrom.

The technique described above provides a single plug around the liner.To effect treatment to a particular zone of a well having a perforateliner it is typically necessary to set two or more such plugs, such thatthere is one plug on either side of the zone in question. Packers canthen be rim into the liner and sealed against the plugs so as to isolatethe zone therebetween and allow a selective treatment to be applied tothat zone. It will be appreciated that if the treatment zone is near thebottom of the well or another plug may only be necessary to set a singleplug to define the zone.

The techniques described above mostly require that the plugging fluid bepumped from the surface to the region in question. In an alternativeembodiment, the fluid can be held in a downhole reservoir near theregion, and pumped through the chamber using a downhole pump. Thislessens the strict rheological requirement of the fluid, and allowsdownhole mixing of two-part fluids or the like--for example, epoxyresins--which can set rapidly in the region without causing problems inthe tubing itself. A downhole source of radiation such as UV or heatmight be provided near the region to trigger or aid the setting of theplugging fluid. Triggering of downhole pumps or sources can be achievedby in situ measurements--for example, the conductivity of fluids passingthrough the tool. An alternative is to use encapsulated cross-linkingagents which can be released by thermal, chemical or mechanicaldegradation.

In a second aspect the invention provides a method of isolating a zoneof a well which is lined with a perforate liner, this method comprising:placing plugs on either side of the zone according to the method of theinvention; and then setting a packer in the liner adjacent each plug.

There are occasions other than when dealing with the special problemsposed by perforate liners when it may be desirable to form a plug in thewell (perhaps in the well itself, or possibly in the annular regionbetween the wellbore wall and some tubing within the wellbore), and thetechnique disclosed herein of employing as the plugging material athixotropic fluid, which is itself a novel concept not hitherto proposedin the Art, may be useful for this purpose. Such occasions includeselective abandonment of a section of the well, as well as the regulatedfill of a washout. Accordingly, in a further aspect the inventionprovides a method of forming a plug in a region in a well, in whichmethod a volume of plugging fluid is pumped into the region,conveniently via suitable packer apparatus such as a pair ofregion-delimiting packers, which volume is sufficient to displacesubstantially all other fluids from the region to be plugged, theplugging fluid being significantly thixotropic.

The plugging fluid is significantly thixotropic--that is to say, itsshear yield strength Tau_(y) (the force required notionally initially tomove a unit contact area block) must be such that under the likelyambient conditions the fluid flows readily when being pumped and yetrapidly gels when pumping stops. Naturally, acceptable values of Tau_(y)depend upon the physical parameters of the well (of the wellbore and ofany tubing therein). In order to flow easily along a 51/2 inch (about 12cm) liner, for example, when the shear rates are high, Tau_(y) shouldpreferably be around 50Pa or less (such a fluid will also flow easilythrough the likely holes in a perforate liner), while to gelsufficiently rapidly and completely within an annulus of between 1 and 4inches (about 2.5 to 10 cm) width, and outside diameter 8 inches (about20 cm), when the shear rates are low, Tau_(y) should preferably bearound 150Pa or greater. Of course, the evolution of Tau_(y) from itslower to its higher value should most desirably occur within a shorttime span suited to the circumstances--and 15 seconds, say, is generallysatisfactory.

The invention will now be described with reference to the accompanyingdrawings, in which: shows a schematic side view of a plugged operationin

FIG. 1 shows a schematic side view of a plugged operation in accordancewith the present invention;

FIG. 2 shows a cross-section on the line AA' of FIG. 1; and

FIG. 3 shows a selective treatment performed in a well which has beenplugged in accordance with the present invention.

Referring to FIGS. 1 and 2, there is shown a horizontal wellbore 10 inwhich a slotted liner 12 has been located. The liner 12 is not cementedto the formation, and fluid can move along the well either inside oroutside the liner 12.

The method of the present invention is performed by running a pair ofpackers 14,16 into the liner 12 from the surface by means of tubing 18.The packers 14,16 are spaced apart in the tubing 18 such that when theyare inflated inside the slotted liner 12 a chamber 20 is defined, therebeing slots 22 in the liner 12 allowing communication between thechamber 20 and the exterior region 24 of the liner 12. A port (notshown) is provided in the portion 26 of the tubing 18 passing betweenthe packers 14,16 in the chamber 20.

In use, the region to be plugged is identified in the conventionalmanner, and the tubing 18 and packers 14,16 are run into the liner 12until they are level with the region 24. The packers 14,16 are theninflated so as to seal against the inner surface of the liner 12. A washfluid can be pumped through the tubing 18 into the chamber 20 throughthe port and then into the region 24 through the slots 22. The chemicalnature of this fluid and the rate of pumping is designed to clean theouter surface of the liner 12 and the wellbore wall and leave them waterwet.

After the wash fluid, a predetermined volume of plugging fluid, usuallycement, is pumped through the tubing 18 into the chamber 20 and theregion 24 outside the liner 12. The theological properties of the fluidand the rate of pumping are chosen to ensure the optimum removal offluids or other material in the region 24 to be plugged. The size of thechamber is made as small as possible, so that the amount of fluidpresent when the plugging fluid is pumped is kept as small as possiblethus reducing contamination of the plugging fluid (also, the smaller thechamber the smaller the amount of plugging fluid that will have to becleaned from inside the liner when the plug has been placed). Once theappropriate volume of cement has been introduced into the region 24,pumping is stopped. The volume is such that the cement does not extendbeyond the limits of the packers 14,16 but fills the region 24 to beplugged completely so as to bond to the borehole wall and the liner 12.The packers 14,16 are then partially deflated, and are moved away fromthe treatment area. Any remaining cement is then pumped out of thetubing 18 and chamber 20. Sufficient volumes of a flushing fluid arethen circulated to ensure removal of unwanted cement. The packers 14,16are then totally deflated, and further flushing fluid is circulated toensure that the liner 12 is left clear. The cement forming the plug isleft to harden.

In FIG. 3 plugs 40,42 are set on either side of a water entry 44 whichis to be sealed. Packers 46,48 are run into the liner or tubing 49. Thepackers 46,48 are set at each plug 40,42, and treatment fluid is pumpedinto the treatment zone 50 to seal off the water entry.

The tubing used to set the packers 14,16 or 46,48 can include a bypassso that fluids can pass up the well past the region at which the plug isbeing set or the zone undergoing treatment. This means that the welldoes not have to be shut in while completing these operations, and soavoids formation damage leading to loss of production from the well.

We claim:
 1. A method of placing a plug in an uncollapsed region arounda continuous perforate liner in a wellbore; said method comprising thesteps of:placing a pair of inflatable packers along a predeterminedlength of said perforate liner so as to seal against an inner surface ofsaid liner where the plug is to positioned, said packers being spacedapart so as to define both a chamber inside the perforate liner and theuncollapsed region to be plugged outside the liner; pumping apredetermined volume of a thixotropic plugging fluid into the chamberand, via the perforate liner, into the region to be plugged, saidplugging fluid being characterized by a gelling time comparable to orshorter than the time taken to displace the fluid into the region and byhaving a gelling strength after said gelling time sufficient toeliminate gravity-induced flow; and removing said packers.
 2. A methodas claimed in claim 1, wherein the volume of the chamber is smaller thanthe volume to be plugged.
 3. A method as claimed in claim 1, wherein theperforate liner is a slotted liner or a pre-packed screen.
 4. A methodas claimed in claim 3, wherein the perforate liner is a slotted linerand the plugging fluid is a cement, or the perforate liner is apre-packed screen and the plugging fluid is a resin.
 5. A method asclaimed in claim 1, in which, prior to pumping in the plugging fluid, awash fluid is pumped through the chamber and region to be plugged.
 6. Amethod as claimed in claim 1, in which, after the plugging fluid hasbeen pumped into the region to be plugged, the packers are moved to alocation separate from the region to be plugged, and a wash fluid iscirculated through the chamber and perforate liner to remove unwantedplugging fluid therefrom.
 7. A method as claimed in claim 1 includingthe steps of:placing tubing through said inflatable packers; and placinga port in said tubing between said packers for the supply of pluggingfluid from said tubing into said chamber.
 8. A method of isolating azone of a well which is lined with a continuous perforate liner along apredetermined length of the well, the method including the stepsof:placing plugs on either side of the zone along a predetermined lengthof the perforate liner, the step of placing plugs on either side of saidzone including the steps of: placing a pair of packers inside theperforate liner where one of the plugs is to be positioned, the packersbeing spaced apart so as to define both a chamber inside the perforateliner and also a region to be plugged outside the perforate liner;pumping a predetermined volume of a plugging fluid into the chamber, andvia the perforate liner, into the region to be plugged, the pluggingfluid being sufficient to displace substantially all other fluids fromthe region to be plugged; and removing said packers.
 9. A method asclaimed in claim 8 wherein the plugging fluid has a gelling timecomparable to or shorter than the time taken to displace the fluid intosaid region and a gelling strength after said gelling time sufficient toeliminate gravity-induced flow.
 10. A method as claimed in claim 9including the further steps of:placing tubing through said packers andplacing a port in said tubing adjacent said packers for the supply ofplugging fluid from said tubing into said chamber.